1. Field of the Invention
The present invention relates to centralizers used to center tools in a tubular or wellbore downhole and more particularly relates to an expandable centralizer which can center a tool in an opening larger in diameter than a diameter through which the unexpanded centralizer can freely pass.
2. History of the Prior Art
When drilling or working on wellbores for use, for example, in oil and gas exploration, it is desirable to be able to increase the diameter of the wellbore at a location within the well. This is typically done below a section of casing and thus a tool capable of being passed through the casing, which has a first diameter, and expanding to cut or mill out a bore having a diameter greater than the first diameter must be used. Such tools are referred to as underreamers.
Underreaming tools commonly have one or more pairs of cutting blades which are movable from a retracted position to an extended position. The tool is maintained in the retracted position so that it may pass through the casing. It is then moved to the extended position and held there by fluid pressure created in the drill string. A disadvantage of these tools is that the two blade arrangement provides 4 point contact on the walls of the wellbore which does not centralize the tool when the wellbore is inclined or horizontal.
Centralizers are known for centering tools in a wellbore. The first type typically comprise a cylindrical body sized to have a diameter approximately equal to the diameter of the wellbore at the location where the tool needs to be centered. Unfortunately these centralizers cannot pass through openings smaller than the diameter of the wellbore at the location where the tool needs to be centered and therefore cannot be used to center underreamers.
A second class of centralizers has a cylindrical body having longitudinally arranged spring bows disposed circumferentially around the body. These centralizers can pass through openings as small as the diameter of the body and then expand, by virtue of the bows, to centralize themselves and a drill string in wellbores of larger diameters. However, due to the weight of the drill string, these centralizers tend to “drop” to the low side of an inclined or horizontal well and thus lose their centering ability in these environments.